core: cleanup in AddVmCommand

- No need for both tempVar & vmDynamic fields in addVmDynamic method,
  thus tempVar is removed
- Replaced size() > 0 with !isEmpty()
- Extract parameters construction for CreateSnapshotFromTemplate command
  to separate method

Change-Id: Idbe9402fc4dd28ac09b014027cc01a880759f360
Signed-off-by: Arik Hadas <ahadas@redhat.com>